Ion sources are commonly used for a variety of applications, including substrate treatment in the form of heating, cleaning, surface etching, and chemically modifying a surface. Ion sources can be used for depositing oxides, diamond-like carbon, and other useful coatings. Ion sources can be used to support vacuum deposition processes and to modify thin film growth. For example, ion sources can be used to densify, crystalize, or chemically react with depositing atoms. Virtually all ion sources are designed to operate in the reduced pressure of a vacuum chamber. Some ion sources can be used as low-thrust, long-running engines for spacecraft acceleration.
In their most basic form, ion sources typically consist of a plasma-forming component with additional components to extract and accelerate ions from that plasma. These additional components typically create electric or magnetic fields within the plasma gas other than fields created by the plasma-forming component. These additional fields exert attraction or repulsion forces on ions and result in acceleration of ions. The additional components also add complexity to the overall ion source.
The majority of ions in plasmas are positively charged. That is, they are ionized atoms or molecules, typically missing one or two electrons. When these ions are accelerated out of a plasma, the resulting ion beam often carries a greater number of positively charged particles than negative particles (e.g., electrons). This electrical imbalance can create problems with vacuum hardware or processes. To correct these situations, often an additional electron source device such as a hot filament thermionic electron emitter or hollow cathode electron source is added to inject charge balancing electrons into the ion beam. These electron sources are commonly referred to as neutralizers. As with the other additional components, the use of a neutralizer adds complexity to the overall ion source.
One example of a plasma-forming component used in ion sources is a hollow cathode. Hollow cathodes include linear hollow cathodes and point hollow cathodes. Linear hollow cathodes are not commonly used in ion sources; point hollow cathodes are occasionally used in an ion source being employed as a thruster (e.g. for spacecraft acceleration), but such point plasma sources are not usually used for coating substrates (e.g., such as for PECVD coating processes). When hollow cathodes are used, similarly to most ion sources, the extraction of ions from hollow cathode plasmas requires attaching additional components such as separately powered electrodes and magnets. These components often add significant degrees of mechanical and process complexity to an ion source. Further, when using a hollow cathode as the plasma-forming component, known ion sources rely on a dedicated anode structure or separate anodic orifice, resulting in further complexity.
Another problem with ion sources is that during dielectric deposition processes, a dielectric coating can build up on the anode structure. Additionally, when ion sources are used in plasma-enhanced chemical vapor deposition (PECVD) processes, the PECVD processes are known for contaminating nearby vacuum chamber hardware including cathode or anode surfaces (for example, contamination may occur from precursor gasses used in these processes reacting with cathode or anode surfaces).
Accordingly, there is a need for an ion source that overcomes these and other disadvantages of known ion sources.